Lambda property value selector as parameter

Question

I have a requirement to modify a method so that it has an extra parameter that will take a lambda expression that will be used on an internal object to return the value of the given property. Forgive my probable incorrect use of terminology as this is my first foray into LINQ expressions!

I have tried searching for an answer, but as I mentioned, my terminology seems to be off and the examples I can find are far too complex or deal with expressions for collection functions such as .Where(), which I am familiar with.

What I have so far (cut down version):

class MyClass
{
    private MyObject _myObject = new MyObject() { Name = "Test", Code = "T" };

    private string MyMethod(int testParameter, ??? selector)
    {
        //return _myObject.Name;
        //return _myObject.Code;
        return ???;
    }
}

I would like to call it something like this:

string result = _myClassInstance.MyMethod(1, (x => x.Name));

or:

string result = _myClassInstance.MyMethod(1, (x => x.Code));

Obviously the parts which I am missing is the selector parameter in MyMethod, how to apply it to the local variable and how to pass the required property into the method when I am invoking it.

Any help would be appreciated, also extra bonus points for a VB.NET solutions as well as unfortunately the final implementation needs to be in our lone VB project!

Solution 1

private string MyMethod(int testParameter, Func<MyObject, string> selector)
{
    return selector(_myObject);
}

When using Func delegates, the last parameter is the return type and the first N-1 are the argument types. In this case, there is a single MyObject argument to selector and it returns a string.

You can invoke it like:

string name = _myClassInstance.MyMethod(1, x => x.Name);
string result = _myClassInstance.MyMethod(1, x => x.Code);

Since the return type of MyMethod matches the return type of your selector delegate, you could make it generic:

private T MyMethod<T>(int testParameter, Func<MyObject, T> selector)
{
    MyObject obj = //
    return selector(obj);
}

I don't know VB.Net but it looks like it would be:

Public Function MyMethod(testParameter as Integer, selector as Func(Of MyObject, String))
    Return selector(_myObject)
End Function

and the generic version would be:

Public Function MyMethod(Of T)(testParameter as Integer, selector Func(Of MyObject, T))
    Return selector(_myObject)
End Function

Solution 2

I will show you a different approach that is very flexible (see DotNetFiddle at the bottom): You can easily write your own LINQ functions to extend existing functions or write your own functions and benefit from the power of LINQ queries.

In this example, I am improving Linq's Distinct function in a way so you can specify a field, which is used for grouping.

Usage (Example):

var myQuery=(from x in Customers select x).MyDistinct(d => d.CustomerID);

In this example the query is being grouped by CustomerID and the first element of each group is returned.

Declaration of MyDistinct:

public static class Extensions
{
    public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query, 
                                                    Func<T, V> f)
    {
        return query.GroupBy(f).Select(x=>x.First());
    }
}

You can see that f, the 2nd parameter, is declared as Func<T, V>, so it can be used by the .GroupBy statement.

---

Coming back to the code in your question, if you have declared

class MyObject
{
    public string Name;
    public string Code;
}

private MyObject[] _myObject = {
    new MyObject() { Name = "Test1", Code = "T"},
    new MyObject() { Name = "Test2", Code = "Q"},
    new MyObject() { Name = "Test2", Code = "T"},
    new MyObject() { Name = "Test5", Code = "Q"}
};

you could use that with the newly defined function MyDistinct as follows:

var myQuery = (from x in _myObject select x).MyDistinct(d => d.Code);

which will return

Name   Code
Test1   T
Test2   Q

or you can use .MyDistinct(d => d.Name) in the query, which returns:

Name   Code
Test1   T
Test2   Q
Test5   Q

Notice that because MyDistinct is declared with the generics T and V, it recognizes and uses the right object types automatically and returns MyObject elements.

---

Advanced usage

Notice that MyDistinct always takes the first element of each group. What if you need a condition defining which element you need?

Here's how you can do it:

public static class Extensions
{
    public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
                                                    Func<T, V> f, 
                                                    Func<IGrouping<V,T>,T> h=null)
    {
        if (h==null) h=(x => x.First());
        return query.GroupBy(f).Select(h);
    }
}

This modification either allows you to use it exactly as before, i.e. by specifying one parameter like .MyDistinct(d => d.Name), but it also allows you to specify a having condition such as x => x.FirstOrDefault(y => y.Name.Contains("1")||y.Name.Contains("2")) as a second parameter like so:

var myQuery2 = (from x in _myObject select x).MyDistinct(d => d.Name,
        x=>x.FirstOrDefault(y=>y.Name.Contains("1")||y.Name.Contains("2"))
        );

If you run this query, the result is:

Name   Code
Test1   T
Test2   Q
null

because Test5 does not meet the condition (it does not contain 1 or 2), you're getting null in the 3rd row.

Note: If you want to expose just the condition, you can have it even simpler by implementing it as:

public static IEnumerable<T> MyDistinct2<T, V>(this IEnumerable<T> query,
                                                Func<T, V> f,
                                                Func<T,bool> h=null
                                                )
{
    if (h == null) h = (y => true);
    return query.GroupBy(f).Select(x=>x.FirstOrDefault(h));
}

In this case, the query would just look like:

var myQuery3 = (from x in _myObject select x).MyDistinct2(d => d.Name,
                    y => y.Name.Contains("1") || y.Name.Contains("2")
                    );

so you don't need to write x=>x.FirstOrDefault(... condition ...).

Try it in DotNetFiddle

Solution 3

in C#

The parameter type you are looking for Func

private string MyMethod(int testParameter, Func<MyClass,string> selector){
    return selector(_myObject);
}

in VB you still want Func the syntax is a little different.

Function MyMethod(ByVal testParameter As Integer, ByVal selector as Func(Of MyClass,string) as string
    return selector(_myObject)
End Function

Comments

[XN16]

It's really that easy?! I feel a little stupid about now! What is the significance of the string in the Func declaration?

[Servy]

@XN16 It specifies the return type of the delegate.

[XN16]

@Servy I assume this would have to be modified if I tried to access another property of a different type?

[Lee]

@XN16 - I've added a brief explanation of the Func type arguments.

[Servy]

@XN16 Well, MyMethod returns a string, so if the delegate didn't match it wouldn't compile. To be able to allow an arbitrary selector you'd need to make MyMethod generic and use the generic argument for both the selector and the return type.

[Mr.Mindor]

@XN16 yes if you are accessing properties of different types you would need multiple overrides for MyMethod. Unless you always want them returned as a string regardless of their type, then you can just change the delegate x => x.NotAString.ToString()

[XN16]

Thanks for the answer, I have now implemented it and it works perfectly!

[kofifus]

it's good to mention that Func<MyObject, T> selector does not guarantee that T is a member of MyObject